Thermocouple safety pilot switch



m. w m m March l, 1949. J. H.. THORNBERY THERMOCOUPLE SAFETY PILOT SWITCH Filed May 28, 1945 lll/lun,

Patented Mara 1, 1949 2,483,218 THERMCQUPLE SAFETY PILOT SWITCH John H. Thornbery, Milwaukee, Wis., assignor to Milwaukee Gas Specialty Company, Milwaukee, Wis., a corporation of Wisconsin Application May 28, 1945, Serial No. 598,358

'This invention relates, in general, to an electric switch, and has particular relation to a thermocouple safety pilot switch.

The device which I shall describe hereinafter in connection with the drawing is particularly adapted for use with a thermocouple safety control device having an electromagnet connected in circuit with a thermoelectric generator and provided with an armature which is held attracted as long as the thermoelectric generator is heated, for example, by a pilot light for a main burner, and moves to retracted position when the pilot light is extinguished.

One of the main objects of the present invention is to provide an improved form of switch having switch operating means adapted to be pressed to move the armature to attracted position without closing the switch, and, thus, without establishing a supply of fuel for the main burner; the closing of my improved form of switch requiring not only release of the switch operating means, but, also, that the armature be held in attracted position by energization of the electromagnet by the heat, for example, ofthe pilot flame on the thermocouple.

Another object of the invention is to provide an improved form of thermooouple safety pilot switch having various features of novelty and various advantages and which is particularly characterized by its simplicity in construction, its relatively few parts, its economy in manufacture, and its effectiveness in use.

Another object of the invention is to provide a device of the class described, comprising an insulating plunger and plunger contact having unitary movement and in which the insulating plunger also has movement relative to the plunger contact, and contact means held out of contact with the plunger contact by' the insulating plunger as long as the plunger and plunger contact are free for unitary movement, the plunger contact being exposed for engagement with the contact meansand the contact means beingireed for movement into closed circuit position engaging the plunger contact by movement of the`= insulating plunger relative to the plunger contact.

Another object of the invention is to provide plunger means comprising a pair of insulating plunger members having a plunger contact engaged in one of the plunger members, with the other plunger member slidableon the plunger contact, with the insulating plunger members and plunger contact having unitary movement in opposite directions, and there being holding means for restraining movement of one of the plunger members and contact means held out of contact with the plunger contact by the insulating plunger member as long as the holding means is inoperative, the plunger contact being exposed for engagement with the contact means and the contact means being freed for movement into closed circuit position engaging said plunger contact by movement of one plunger member relative to the other plunger member when said other plunger member is restrained against.- movement by the holding means.

Another object of the invention is to provide plunger means comprising inner and outer coaxial insulating plunger members having a plunger contact engaging at one end in the outer end of the inner plunger member, Iwith the other plunger member slidable on the outer end of the plunger contact, the insulating plunger members vand plunger contacts having unitary movement in opposite directions, and there being holding means for restraining movement of the inner plunger member, and contact means yieldingly urged toward the axis of the plunger means with a spring disposed within the outer plunger member and between said plunger member and the plunger contact for separating the outer plunger member from the inner plunger member to expose said plunger contact for engagement with l said contact mea-ns and to free the contact means for movement into closed circuit position engaging the plunger contact when the inner plunger member is restrained against movement by the holding means, and a spring operable to move the inner plunger' member relative to the outer plunger member upon release of the holding means and maintaining, except when the inner plunger member is restrained against movement by the holding means, said inner plunger member in endwise abutting engagement with the outer plunger member, with the plunger contact enclosed and the contact means held out of contact therewith by the insulating plunger members. Another'obiect of the invention is to provide a pair of spring contacts yieldingly urged toward each other to closed position, plunger means slidable between the contacts and comprising a pair of insulating plunger members enclosing a bridging plunger contact and holding the contact springs out of contact therewith, and means operable to separate the insulating plunger members to expose the bridging plunger contact for engagement with the spring contacts and to i'ree the spring contacts for movement into closed circuit position engaging said bridging plunger contact.

Another object of the invention is to provide a device of the character set forth wherein the last mentioned means comprises a spring enclosed within one of the insulating plunger members.

Another object of the invention is to provide a device of the character set forth wherein the opposed ends of the insulating plunger members 60 are marginally tapered.

Another object of the invention is to provide a device of thecharacter set forth wherein the spring contacts are carried by an insulating terminal block supported in position laterally of the plunger means and having terminals for connecting the spring contacts in an electric circuit.

Another object of the invention is to provide a device of the character set forth wherein the spring contacts and plunger means are mounted withina generally U-shaped frame, with one of the plunger members projecting, for axial movement, through an opening in one end of the frame.

Further objects and advantages of the invention will be apparent fromjhe following detailed description, taken in connection with the accompanying drawing.

In the drawing; A

Figure 1 is a schematic view showing one application of a switch embodying the present invention;

Figure 2 is a longitudinal sectional view taken on the line 2-2 of Figure 3;

Figure 3 is a transverse sectional view taken on the line 3-3 of, Figure 2; I

Figure 4 is atop plan view of the switch showing the plunger and contact means in the positions which they occupy when the armature is in retracted position; and

Figure 5 is a view similar toFlgure 4 showing in full lines the plunger means pressed inwardly to position the armature inattracted position, the outer insulating plunger member and contacts being shown in dotted lines in the positions .which they occupy when the armature is held in attracted position and the outer plunger member is released.

In Figure 1 of the drawing, a gas or other fuel burner of any suitable or preferred type is illustrated at i0. The burner i0 may be the burner of a room or space heater, water heater, oor furnace, gas range, or any other burner. A gas or fuel supply pipe Ii leads to th'e burner I0 for the delivery of gas or other fuel thereto-for example,

A pilot burner 23 isplaced in juxtaposition to the main burner I0 and provides a pilot light or flame 24 for lighting the main burner. A pipe 25, connected to the fuel supply pipe il anterior of the valve I3, or otherwise as desired, supplies fuel for the pilot burner 23.

The switch 20 has a generally U-shaped supporting bracket 23. The U-shaped magnet frame 21 is secured to a terminal bushing 29, for example, in the manner more fully disclosed in Clarence Wantz U. S. Patent No. 2,318,823, issued May 11, 1943. The coil 29 of the electromagnet is wound around the legs of the magnet frame 21. The bushing 2B', the shank' of which extends through an opening in the adjacent end flange 30 of the bracket 23, is shown as externally' threaded for threaded engagement with a nut 3| by means of which the bushing 29 is clamped to the flange 30. A sealing gasket 32 is preferably placed between the head or flange at the inner end of the bushing and the flange 30.

A thermocouple, indicated at 33, is placed in position so that the hot junction 33 thereof will be heated by the pilot flame 2l as long as the pilot flame is burning. While the thermocouple and leads therefor are shown as similar to the thermocouple and leads more fully disclosed in Oscar J. Leins Patent No. 2,126,564, issued August 9, 1938, it is to be understcodthat the thermocouple and its leads may be of any other suitable or preferred form Within the scope of the through a mixing chamber i2 .to which air is admitted in the usuabway, as well understood in the art.

An electroresponsive valve i3, such as a solenoid or motor valve, is interposed in the fuel supply pipe Il. The valve I3 is shown as comprising a valve body; i4 and a hood lhaving any suitable power operated lvalve actuating means therein. -A step-down transformer I6 supplies power for the operation of the valve I3. This transformer comprises a line voltage primary l1 and a low voltage secondary I9. The primary i1 is connected to line wires I9 leading to any suitable source of current or power (not shown). The valve actuating means for the valve i3, theswitch of the present invention, which is designated in its entirety at 20. and a thermostat, such as a room thermostat 2|, are shown connected in series circuit relationinethe secondary circuit 22. Y

For purposes of the present disclosure it may be assumed that the valve i3 is of a character to lbe actuated to open position by closing of the circuit 22, and to be held in open position to supply fuel tothe burner I0 as long as the circuit 22 is closed. The valvel i3 may have spring means or other suitable means for actuating the valve to closed position to shut oi! the supply of fuel to the burner i0 when the circuit 22 is opened or otherwise disabled.

present invention.

For the purpose of the present description, suffice it to state that the particular thermocouple selected for illustration comprises an outer tubular metallic thermocouple element and an inner metallic thermocouple element of diierent thermoelectric characteristics. The thermocouple elements are joined to form the hot junction 38; An outer tubular lead conductor 31 is connected to the outer thermocouple element, for

example, through a sleeve 33. The inner lead conductor 38, enclosed by and insulated from the outer tubular lead conductor 31, is connected to the inner thermocouple element.

A quick detachable or removable connection is preferably provided between the electromagnet and the ends of the leads 31 and 39 opposite the ends which are connected to the thermocouple elements. The inner lead 39 has a connector cone 40 which seats in the correspondingly recessed, outer end of the terminal tip 4I, and is clamped in contact therewith by a connector sleeve 42 which screws into the outer end of the bushing 28. The adjacent end of the outer tubular conductor 31 is connected in circuit with one side of the coil 29 through the connector sleeve 42 and the bushing 23, and the other side of the coil 29 is connected to the inner lead conductor through the terminal tip 4I, the parts being suitably insulated. f

The electromagnet is enclosed within a housing 4l, the outer end `of which is turned over or .secured at 46 to the base flange at the inner 'plunger member with a coaxial insulating 55 and a plunger contact 53, constitutes the plunger means for resetting the switch, as will be presently described. A coil spring 51, interposed between the spring seat member 52 and a second spring seat member 58 on the plunger member 54. moves the armature 48 to retracted position and the plunger means to the position shown in Figure 4 when the armature 48 is released from the electromagnet.

The contact means comprises a pair of spring :contacts yBI) the free ends of which are yieldingly urged to closed circuit position 'toward the axis of the plunger means. The opposite ends of the contacts 80 are secured, for example, by terminal sleeves 8i to an insulating terminal block 52.

member 54 which,

y Terminal screws 53, screwed into the terminal sleeves 8|, are adapted for connecting the spring contacts 80 to the adjacent conductors of the electric circuit 22. The terminal block 82 is secured tothe supporting bracket or frame 28 in position laterally oi' the plunger means. A grommet is provided at 16 through which the conductors may extend for connection to the terminals 8|. An angular piece of insulation may be provided as indicated at 15.

The plunger contact 58 is illustrated in the form of a post or pin contact engaged at one end in the outer end of the inner insulating plunger member 54. The outer insulating plunger member 55 is slidable on the outer end of the plunger contact 58. A spring 85, disposed within the outer plunger member -55 and between said plunger member 55 and the plunger contact 58, is operable to separate the outer plunger member 55 from the inner plunger member 54 to expose the plunger contact 58 for engagement with the contact springs 80 and to free the contacts 50 for movement into closed circuit position engaging the bridging plungercontact 58 when the `armature 48 is held in attracted position by the electromagnet and the outer' plunger member 55 is released.

The outer end of the outer insulating plunger member 55 projects, for axial movement, through an opening 88 in the adjacent end of the frame 28,' the projecting end of the plunger member 55 constituting a button which is exposed for manipulation, for example, with the thumb or fingers, to reset the device. A stop ring 1li, secured in an annular groove in the plunger member 55, cooperates with the adjacent end of the frame 28 to limit the outward movement of the plunger member 55.

The free ends of the spring contacts 80, which engage theaplunger contact 58, are preferably of generally V-shaped section. Vas illustrated in the drawing. The opposed ends of thecoaxial insulating .plunger members 54 and f55, which cooperate with these V-shaped ends of the contacts 50, are preferably tapered marginally as is also illustrated inthe drawing.

With the parts in the positions shown in Figure 4, the following/sequence is followed in operating the system illustrated in the drawing:

The plunger means is pressed inwardly from the position shown in Figure 4. for example, by engaging the projecting end of the insulating plunger member 55 with the thumb or fingers and pressing the same inwardly. The plunger member 55, by its endwise abutting engagement with the plunger member 54, moves the plunger members 54 and 55 inwardly as a unit, moving the armature 48 by rectilinear movement from its retracted position to its attracted position,

valve I3.

In this inward movement the contacts 83 ride from as shown in Figure 2. of the plunger means the plunger member 54 member 55 retained out contact 58.

Upon ignition of the pilot burner 23, the pilot ame 24, by heating the hot junction 38 of the thermocouple 35, energizes the electromagnet suiiiciently to hold the armature 48 in the attracted position shown in Figure 2. Whenthe plunger member 55 is released, with thearmature 43 held in attracted position by the electromagnet, the spring 85 moves the plunger member 55 outwardly, thus separating it from the insulating plunger member 54 as shown in Figure This separation of the from the plunger member 54 exposes the plunger contact 55 for engagement with the contacts 80 and. at the same time, frees the contacts for snap movement by their own resilience into closed circuit position engaging the plunger contact 58 which then bridges the contacts 80 and completes, 'at the switch I2li, the circuit 22 for the Assuming that the circuit is also closed at the thermostat 2|, the valve I3 is opened by the completion of the circuit 22 of fuel to the main burner I 0 which is then ignited by the pilot flame.

From the foregoing it will be apparent that the switch 20 closes the circuit 22 not by the inward movement of the plunger means or by the armature resetting operation, but only if and after the armature 48 is held in attracted position by the electromagnet and the outer plunger member 55 is released. The closing of the circuit 22 and opening the valve I3 for the supply of iuel to the main burner is thus prevented except when the pilot burner is ignited so that it will ignite the fuel which is delivered to the main burner. n

If the heat of the pilot flame on the thermocouple has not generated sufficient E. M. F. to hold the armature 48 in attracted position, the spring 51 will return the insulating plunger mem- -bers 54 'and 55 and plunger contact 56 as a unit to the position shown in Figure 4. In this unitary outward movement of the plunger means the contacts 68 ride from the insulating plunger member 55 to the insulating plunger member 54, and are thus retained out of contact with the bridging plunger contact 56. 'The switch 20 thus remains open and the valve I3 remains closed.

If, after the parts are set as shown in Figure 2 and the pilot burner is lighted to hold the parts in the positions shown in this figure, the pilot flame is extinguished, the armature 48 is released. The spring 51 then moves the armature 48 to retracted position and drives the insulating plunger member 54 between the switch members 80 to open the switch. 'Ihe tapered outer end of the plunger member 54, in conjunction with the Eenerally V-shaped formation of the adjacent ends of the spring contacts 60, facilitates entry of the plunger member 54 between the contacts to sep- -arate and hold the same out of contact with the plunger contact 58.

The Iembodiment lof the invention shown in the drawing is for illustrative purposes only, and it is to be expressly understood that the drawing and the accompanying specification are not to be construed as -a deilnition oi' the limits or scope oi' the invention, reference being had to the appended claims for that purpose.

(full lines, Figure 5), and are thus o! contact with the bridging plunger (Figure 4) to the plunger plunger member 55 for the supply I claim: i

i. In a thermoelectric safety device, in combination, an electromagnet, a thermoelectric generator connected in circuit with said electromagnet. an armature adapted to be heldin attracted position by energization of the electromagnet by l,

the'thermoelectric generator and movable to retracted position upon deenergization of the electromagnet, the combination therewith oi' a' pair of contact members yleldingly urged to closed circuit position, and an insulating' plunger and plunger contact having unitary movement between said contact members in one direction for resetting the amature to attracted position and movement in the opposite direction upon movement of the armature to retracted position, said insulating plunger having movement relative to the plunger contact and holding said contact members out of contact with the plunger contact at all times except when the armatureis held in attracted position by the electromagnet, whereupon movement of the insulating plunger relative`to the plunger contact exposes said plunger contact for engagement with said contact members and freessaid contact members for movement into closed circuit position engaging said plunger contact.

2. In a thermoelectric safety device of the class described. in combination, an electromagnet, a thermoelectric generator connected inV circuit with said electromagnet,an armature adapted to be held in attracted position by energization ofv the electromagnet by the thermoelectric genera-k tor and movable to retracted position upon deenergization of the electromagnet, the combination therewith of a pair of contact members yieldingtact means and to free said contact means for movement into closed circuit position engaging said plunger contact when the armature is held in -attracted position by the electromagnet and the outer plunger member is released, and a spring operable to move the armature to lretracted position upon deenergization of the electromagnet and maintaining, except when the armature is held in attracted position by the electromagnet, said insulating plunger members in endwise abutting engagement enclosing said plunger contact and holding said contact means out of contact with the plunger contact, for uni` tary movement of the plunger members andl plunger contact in one direction for resetting the armature tov attracted position and in the opposite direction when the armature is free for movement to retracted position.

.4. In a thermoelectric safety device of the class described, in combination, an electromagnet, a

thermoelectric generator connected in circuit 4 with said electromagnet, an armature adapted to be held in attracted position by energization oi the electromagnet by the thermoelectric generator and movable to retracted position upon deenergization of the electromagnet, thecom-bination "therewith of contact means yieldingly urged to ly; urged to closed position, and plunger means comprising a pair of insulating plungerv members having a plunger contact engaged in one of said plunger members with the other plunger member slidable on said plunger cont-act, said plunger means having movement between said contact members in'one direction for resetting the armature to attracted position and movement' in the opposite direction upon movement of the armature to retracted position, said insulating plunger members holding said contact members out o! contact with the plunger contact at all times except when the armature is held in attracted position therewith of contact means yieldingly urged l to closed circuit position, plunger means' comprising inner and outer coaxial insulating plungerV members having a plunger contact engaging at one end in the outer end of the inner plunger member with the outer plunger member slidable on the outer end ofsald plunger contact, a spring disposed within said outer plunger member and between said plunger member and said plunger contact for separating the outer plunger member from the inner plunger member to expose said plunger contact for engagement with said conclosed circuit position, plunger means comprising inner and outer coaxial insulating plunger members `having a plunger contact engaging at one end in the outer end of the innerl plunger member with the outer plunger member slidable on the outer end of said plunger-- contact, -a spring disposed within-said outer plunger member and between said plunger member and ,said plunger contact for separating the outer. plunger member from the inner plunger member to expose .said plunger contact for engagement with said contact means and to free said contact means for movement into closed circuit position engaging said plunger contact when the armature -is held in attracted position by the electromagnet and the outer plunger member is released, and a spring operable to move the armature to retracted position upon deenergi'zation of the electromagnet and maintaining, except when the armature is held in attracted position by the electromagnet, said insulating plunger members in endwise abutting engagement enclosing said plunger oon-tact and holding said contact means out of contact with the plunger contact, for unitary movement of the -plunger members and plunger contact in one direction for resetting the armature to attracted position and in the opposite direction when the armature is iree for movement to retracted position, movement of the armature from attracted position to retracted position with the outer plunger member in released position causing the inner plunger member to separate and hold Ithe contact means from engagement with said plunger contact.

5. In a thermoelectric safety device of the class described, in combination, an electromagnet, a

thermoelectric generator connected in circuit with saidelectromagnet, an armature adapted to be held in attracted position by energization of the electromagnet by the thermoelectric generator and movable to retracted position upon deenergization of the electromagnet, the combination therewith of contact means yieldingly urged to vclosed circuit position, plunger means comprising inner and .outer coaxial insulating plunger -members having a plunger contact engaging at one end in the outer end of the inner plunger member with the outer plunger member tact, a spring disposed within said outer plunger member and between said plunger member and said plunger contact for separating the outer plunger member from the inner plunger member to expose said plunger contact for engagement with said contact means and to free said contact means for movement into closed circuit position engaging said plunger contact when the armature is held in attracted position by the electromagnet and th outer plunger member is released,

and a spring operable to move the armature to retracted position upon deenergization. of the electromagnet and maintaining, except when the armature is held in attracted position by the electromagnet, said insulating plunger members in endwise abutting engagement enclosing said plunger contact and holding said contact means out o! contactwith the plunger contact, for unitarymovement oi.' the plunger members and plunger contact in one direction for resetting the armature to attracted position and in the opposite direction when the armature is free for movement to retracted position, movement of the armature from attracted position to retracted position with the outer plunger member in -released'position causing the inner plunger member to separate and hold the contact means from engagement -with said plunger contact, said ilrst mentioned contact means comprising a pair of spring `contacts yieldingly urged toward each other to closed circuit position and the plunger means being positioned ior'operation between said spring contacts.

6. In a device o! theclass described, in combination, an insulating coaxial plunger and a' plunger contact having unitary movement, said insulating plunger also having sliding movement on and relative to said plunger contact. and contact means disposed on opposite sides of said plunger contact and held out of contact with the plunger contact by said insulating plunger as long as the plunger and plunger contact are free for unitary movement, said plunger contact being exposed for engagement with said contact means and said contact means being freed for movement into closed circuit position engaging said plungervcontact by movement of the insulating plunger relative to said plunger contact.

7. In'a device .of the class described, in combination, plunger means comprising a pair of coaxial. insulating plunger members having a coaxial plunger contact engaged in one of said plunger members with the other plunger member slidable on said plunger contact, said insulating plunger members and plunger contact having unitary movement in opposite directions, holding means for`restraining movement of one of said plunger members, and contact means disposed on opposite sidesv of said plunger contact and held out of contact with the plunger contactA by said insulating plunger members as long as said holding means is inoperative, said plunger contact being exposed for engagement with said contact means and said contact means being freed for movement into closed circuit position engaging said plunger contact by movement of one plunger member relative to the other plunger member when said other plunger member is restrained against movement by said holding means.

8. In a device of the class described, in combination, plunger means comprising inner and outer coaxial insulating plunger members having .a plunger contact engagingl at one end lin the 1 outer end o! the -inner plunger member, with the other plunger member slidable on the outer end of said plunger contact, said insulating plunger members and plunger contact having unitary movement in opposite directions, holding means for restraining movement of said inner plunger member, contact means yieldingly urged toward the-axis of said plunger means, a spring disposed within said outer plunger member and between l0 said plunger member and said plunger contact for separating the outer plunger member from the inner plunger member to expose said plunger contact for engagement with said contact means and to ireesaid contact means for movement into closed circuit position engaging said plunger contact when said inner' plunger member is restrained, against movement by said holding means, and a spring operable to move said inner plungermember relative to said outer plunger memberupon release of said holding means and maintaining, except when the -lnner plunger member,

is restrained against movement by said holding means, said inner plunger member in endwise abutting engagement with said outer plunger member with said plunger contact en- .A closed and said contact means held out oi congact therewith by said insulating plunger memers.

9. In a device of the class described, in combination, plunger means comprising inner and outer coaxial insulating plunger members having a plunger contact engaging at one end in the outer end of the inner plungerl member, with the other plunger member slidable on the outer end of said plunger contact. said insulating plunger members and plunger contact having unitary movement in opposite directions, holding means for restraining movement of said inner plunger member, contact means yieldingly urged 40 toward the axis of said plunger means, a spring disposed within said outer plunger member and between said plunger member and said plunger contact for separating the outer plunger member from the inner plunger member to expose said plunger contact for engagement with said contact means and to free said contact means for movement into closed circuit position engaging said plunger contact when said inner plunger memberlis restrained against movement by said vholding means, and a spring operable to move .said inner plunger member relative to said outer plunger member upon release of said holding means and maintaining, except when the inner plunger member is restrained against movement "n by saidholding means, said inner plunger mem ber in endwise abutting engagement with said outer plunger member with said plunger contact enclosed and said contact means held out of contact therewith by said insulating plunger ,-members, said contact means comprising a pair of spring contacts yieldingly urged toward each other to closed circuit position and the plunger means being positioned for operation between 05 said spring contacts.

l0. In combination, a pair of spring contacts yieldingly urged toward each other to closed position, plunger means slidable between said contacts and comprising a pair of insulating plunger members enclosing a bridging plunger contact and holding said spring contacts out of contact therewith, and means operable to separate said insulating plunger members to expose said bridgv ing plunger contact for engagement with said spring contacts and to free said spring contacts ll for movement into closed circuit position engaging saidl bridging plunger contact.

11. A device according to claim wherein'the last mentioned means comprises a spring enclosed within one of said insulating plunger members.

f 12. In a device of the class' described, in combination, al pair of contacts at least one of which is yieldingly urged toward the other to closed circuit position, insulating plunger means mov`` able into and from position between said contacts and, when positioned between said contacts, hold'- ing them in open circuit position, and a plunger contact positioned substantially at the axis of said insulating plunger means and extending from the end oi said insulating plunger means for engagement with said pair of contacts when said insulating plunger means is out of position between said pair of contacts, said plunger contact being o! a diameter less than the-diameter of said insulating plunger, with the yieldingly urged contact arranged to engage said plunger contact radially inwardly of the outer periphery of the insulating plunger, said insulating plunger, due to its larger diameter, springing said yieldingly urged contact outwardly from engagement with said plunger contact upon movement between said pair of contacts.

13. In a device of the class described, in combination, a pair of contacts at least one oi which v is yieldingly urged toward the other to closed circuit position, an insulating plunger member. movable into and from position between said con-V tacts and, when positioned between said contacts, holding them separated and in open circuit position, and a coaxial plunger contact positionedat the axisl of said insulating plunger member, said insulating plunger member having axial movement on and relative to said plunger con-` tact, and spring means interposed between said plunger contact 'and said insulating plunger mem-- ber for moving said insulating plunger member relative to said plunger contactv and out of position between said pair of contacts whereby to expose said plunger contact to said pair of contacts and to free said pair oi contacts for movement to closed circuit position engaging said plunger contact.

14. In a device of the class described, in combination, a pair of contacts at least one`of which.

is yieldingly urged toward the other to closed circuit position, an insulating plunger member mov' able into and from position between said contacts contacts.

able with said insulating plunger member tolle between said pair of contacts for engagement vthereby when said insulating plunger member is out of position between said pair of contacts.

said plunger contactbeing oi a diameter less'than the diameter of said insulating plunger, with the yieldingly urged contact arranged to engage said plunger contact radially inwardly oi' the outer periphery of the insulating plunger, said insulating plunger, due to its larger diameter, springing said yieldingly urged contact outwardly from engagement with said plunger contact upon move ment between said pair of contacts. 15. In a device of the'class described, in combination. a pair of contacts atleast one oi' which is yieldingly urged toward the other to closed circuit position, a rst insulating plunger member movable into and from position between said contacts and, when positioned between said contacts, holding them separated and inl open circuit position, a plunger contact engaged at one end in the adjacent end of said rst insulating plllnger member and extending therefrom, and a second insulating plunger member slidable on the extending end of said plunger contact into and from position between said ilrst contacts' and, when positioned between said rst contacts, holding them separated and in open circuit position, said plunger contact being exposed and said rst contacts being freed for movement into closed circuit position engaging /said plunger contact onlywhen said first and second insulating plunger members are out of position between said pair of a JOHN H. 'rHoaNBa'am REFERENCES CITED.

yThe following references are of record in the lle of this patent:

UNITED STATES PATENTS J2,293,382 Care et al Aug. 18. 1942 

